I got into yoga at 17, after meeting a practitioner in Crimea and reading Yoga Sutras of Patanjali and Bhagavad Gita. My style is based mainly on Russian translations of ancient texts, and involves full relaxation in a pose with concentration with specific air circulation. I have practiced raja-yoga (for meditation), pranayama (breathing), hatha-yoga (asanas and bandhas), elements of Jain Dharma, some Tao yoga, Kundalini (to convert libido), Ch'an yoga, and Zen yoga, as well as similar practices, like Nishi. Western variations of yoga are usually not my thing.
Most important concepts in yoga for me are Smadhi (liveration), Ahimsa (non-harming), and Satya (truthfulness), Svadhyaya (self study), Dhyana (contemplation). The word "yoga" could be translated as unity (of senses and of self) or tool (for steadying the mind).
Yoga is a group of physical, mental, and spiritual practices aimed at controlling the mind. Yoga was first mentioned in Rig Veda and Upanishads, in about 4th century BCE.
Pleasure and suffering arise as a result of the drawing together of the sense organs, the mind and objects. When that does not happen because the mind is in the self, there is no pleasure or suffering for one who is embodied. That is yoga. - Vaisesika sutra
When the five senses, along with the mind, remain still and the intellect is not active, that is known as the highest state. - Katha Upanishad
Be equal minded in both success and failure. Such equanimity is called Yoga. - Bhagavad Gita
Know that which is called yoga to be separation from contact with suffering. - Bhagavad Gita
Yoga is the calming down the patterns of mind. - Yoga Sutras